1. Field of the Invention
This invention relates to survey procedures, and in particular to satellite based positioning systems and a method of using a pole mounted antenna to measure the position of a ground point. The method is especially, though not only, suitable for use in kinematic surveying techniques where the position of a roving antenna/receiver can be obtained with an accuracy in the order of 2 cm or better.
2. Art Background
Traditional surveying involves two operators working with a theodolite and range pole, or a more complex optical electronic "total station". One operator generally positions the theodolite over a known control point while the other holds the range pole at a series of unknown points whose positions are to be determined. A prism mounted on the range pole is sighted through the theodolite and accurate angular and distance measurements to the prism are taken at each point. The positions of the points can then be determined by trigonometry.
An approximately analogous process takes place in modem satellite based surveying. Current techniques involve a reference antenna/receiver located over a known point and a single operator who moves about with a roving antenna/receiver, or "GPS total station". The operator stops on various unknown points to record position information in a data collector using signals transmitted by a minimum number of satellites which are above the horizon. The roving antenna is also carried atop a range pole which is held by the operator, although the roving antenna need not be within sight of the reference antenna. A vector or base line is determined from the reference site to the rover.
In real time techniques, an actual position is determined and recorded at each point during a survey. Other techniques require post processing, in which data from both the reference and roving receivers is recorded for analysis and determination of actual position coordinates later. Most techniques are also either differential or kinematic. In kinematic surveying at least four satellites must be in view of each antenna at all times and centimeter level accuracy can currently be obtained. Differential surveying allows satellites to be temporarily obscured by obstructions between measurement points, and can provide submeter accuracy, which is sufficient for many purposes. Actual positions are calculated as latitude, longitude and height with reference to the global ellipsoid WGS-84 (World Geodetic System 1984). Local northing, easting and elevation co-ordinates can then be determined by applying an appropriate datum transformation and map projection.
The satellite positioning system most commonly in use today is the Global Positioning System (GPS) although others such as the Global Orbiting Navigation System (GLONASS) are also in use or under development. Some land based systems which simulate satellite systems are also being developed. GPS is based on a constellation of twenty four satellites operated by the US Department of Defense. The satellite positions are monitored closely from earth and act as reference points from which an antenna/receiver in the field is able to determine position information. By measuring the travel time of signals transmitted from a number of satellites, the receiver is able to determine corresponding distances from the satellites to the antenna phase center, and then the position of the antenna by trilateration. A minimum number of satellites must be visible to the antenna above the horizon.
Surveyors require a position measurement for the ground point below the roving antenna (or below the prism in traditional procedures) rather than the position of the antenna itself. It is the ground point positions which are required when staking out an area for residential development, for example. However, it is not usually possible to place the antenna directly on the ground point because of signal reflection and satellite obstruction effects, and a separate measurement of the antenna height on the range pole must normally be made. The range pole must also be oriented vertically over the ground point for up to a minute. Experience has shown that manual mistakes made by surveyors when placing and orienting the pole are the most common source of error in satellite based surveying techniques. Departures of the pole from verticality over the ground point are particularly significant.
To reduce these errors in traditional surveying, and improve the reliability of survey measurements, two prisms are sometimes placed along the range pole so that a vector towards the ground point may be determined by the theodolite. The range pole must still be held immobile by the second operator for an appreciable period and the process is not particularly convenient. Use of two antennas in a similar fashion with satellite based techniques is also possible but cumbersome. Instead, most range poles currently incorporate a spirit level device or "bullseye bubble" to provide a visual check on verticality for the surveyor. These are susceptible to damage in the field, and surveyors are not necessarily vigilant throughout the dozens or possibly hundreds of points which are measured during a typical work period. Other poles may incorporate an electronic tilt sensor and compass to allow a check on verticality, but these are relatively expensive and the compass is subject to local magnetic influences.